Symbol Boundary Detection

ABSTRACT

A symbol boundary in a data packet having a guard interval preceding a preamble having a predetermined sequence of symbols is detected by receiving a signal representing a data packet; sampling the received signal at a sampling rate; estimating channel impulse responses from a set of samples in dependence on the predetermined sequence of symbols of the preamble; determining an energy value for each of a plurality of windows of channel impulse responses, each of the windows corresponding to W number of consecutive samples, the energy value for each of the windows being indicative of the total energy associated with the channel impulse responses of that window; determining which of the windows has the greatest energy value; and identifying the earliest sample of the consecutive W samples in said determined greatest energy window, the earliest sample being indicative of a symbol boundary for the preamble.

BACKGROUND OF THE INVENTION

This invention relates to detecting the symbol boundary in orthogonalfrequency division multiplexing (OFDM) signals, such as IEEE 802.11 fora Wireless Local Area Network (WLAN) OFDM signals.

OFDM systems use multi narrow-band sub-channels for improved toleranceto multipath delay. The sub-channels are orthogonal to each other toprevent inter-carrier interference (ICI). One problem with OFDM systemsis that they can be very sensitive to synchronization errors. A wrongestimation of a symbol boundary in OFDM leads to increased inter symbolinterference (ISI) and ICI, which degrades the performance of the OFDMsystem. Thus it is desirable to provide an accurate estimation of asymbol boundary.

A WLAN OFDM data packet has a standard field structure. As an example,the field structure of a packet used in 802.11a/g is shown in FIG. 1.The data packet 100 includes a short training field (STF) 101, a longtraining field (LTF) 102, a signal (SIG) field 103 and a rest of packet(ROP) field 104.

The STF 101 includes a set of ten identical short preambles S1, S2, . .. , S10, each having a duration of 0.8 μs. The STF 101 is followed bythe LTF 102. The LTF 102 includes a double guard interval (DGI) and twoidentical long preambles L1 and L2. DGI is of 1.6 μs duration. The DGIis followed by the two identical long preambles L1 and L2, each having atime duration of 3.2 μs. The samples of the DGI are a copy of thesamples in the last 1.6 μs duration of L1 or L2. The LTF 102 is followedby the SIG field 103, which includes L-SIG (non-high throughput (non-HT)signal field) for 802.11a/g, or L-SIG (non-HT signal field) and HTsignal field (HT-SIG) for 802.11n HT transmission, or HT-SIG for 802.11nGreenField transmission, or L-SIG and very high throughput signal field(VHT-SIG-A) for 802.11 ac VHT transmission.

The ROP field 104 may include additional STF and LTFs and signal fieldsdepending on the type of transmission. The ROP field 104 includes thepayload data, which is represented as data symbols of 3.2 μs duration.Each data symbol is preceded by samples of guard interval duration fromthe end of the data symbol. Guard interval (GI) of 0.8 μs (long GI) or0.4 μs (short GI) is used in WLAN OFDM transmission.

For extracting payload data from the data packet 100, the correctboundary of OFDM symbols must be known. The symbol boundary can be foundusing the known information of the STF and the LTF of the receivedpacket. Traditionally, two methodologies have been used to find thesymbol boundary. The first method finds the boundary between S10 and DGI(the S10-DGI boundary), and the second one finds the boundary betweenDGI and L1 (the DGI-L1 boundary).

The S10-DGI boundary is usually found using an auto-correlation scheme.However, due to high noise and low Signal to Noise Ratios (SNRs), theboundary position determined using such schemes is offset from thecorrect boundary due to the poor correlation metric of short preamblesat low SNR. Thus, the timing variance of the auto-correlationsynchronization scheme is large and may degrade performance of the OFDMsystem. Accordingly, such auto-correlation synchronization schemes areused to achieve only a coarse estimate of the S10-DGI boundary.

The DGI-L1 boundary can be found using a fine symbol boundary (FSB)algorithm, such as the FSB algorithm in Schmidl, T. M. and Cox, D. C.,“Robust Frequency and Timing Synchronization for OFDM”, IEEE Trans.Commun., vol. 45, no. 12, pp. 1613-1621, December 1997, which uses theauto-correlation based on the long preambles. However, this algorithmsuffers from high latency, low accuracy for low SNRs and does not coverthe precursors of the channel.

SUMMARY

According to a first aspect of the present disclosure there is provideda method for detecting a symbol boundary in a data packet comprising aguard interval preceding a preamble having a predetermined sequence ofsymbols, the method comprising: receiving a signal representing a datapacket; sampling the received signal at a sampling rate; estimatingchannel impulse responses from a set of samples in dependence on thepredetermined sequence of symbols of the preamble; determining an energyvalue for each of a plurality of windows of channel impulse responses,each of the windows corresponding to W number of consecutive samples,the energy value for each of the windows being indicative of the totalenergy associated with the channel impulse responses of that window;determining which of the windows has the greatest energy value; andidentifying the earliest sample of the consecutive W samples in thedetermined greatest energy window, the earliest sample being indicativeof a symbol boundary for the preamble.

The plurality of windows may comprise successive windows of W samples,each successive window being offset by one sample from a previous windowin the succession.

W may be dependent on the sampling rate and a duration of the guardinterval.

The guard interval duration may be 0.8 μs or 0.4 μs.

The data packet may further comprise a short training field precedingthe guard interval, the method may further comprise estimating aboundary between the short training field and the guard interval, theset of samples being taken from the estimated boundary for a periodcorresponding to the duration of the preamble.

The received signal may comprise a first signal transmitted from oneantenna and one or more other signals transmitted from respective one ormore other antennas, the other signals being the same as the firstsignal and shifted by predetermined amount of time up to a maximum shiftequal to −d, the method further comprising the steps of: identifying asample n_(Igi) which is the earliest sample in the window of channelimpulse response of duration W having the greatest energy; calculating asample range in dependence on sample n_(Igi), a peak sample n_(max) anda W_(s) number of samples corresponding to the duration of a short guardinterval; determining an energy value for each of a plurality of secondwindows of channels impulse response, each second window correspondingto W_(s) number of consecutive samples, the plurality of second windowsbeing within the sample range, the energy value for each second windowbeing indicative of the total energy associated with the channel impulseresponses of respective W_(s) samples; determining which second windowhas the greatest estimated energy value; identifying the earliest sampleof the consecutive W_(s) samples in the determined second window, theearliest sample for the determined second window being indicative of asymbol boundary.

W may correspond to a number of samples for the duration of a long guardinterval.

The step of calculating a sample range may comprise: identifying thechannel impulse response having the greatest energy as n_(max); anddetermining if n_(max) is for the first signal or the one of the othersignals in dependence on n_(Igi) and W_(s).

The step of calculating the sample range may comprise setting the startof the range as n_(c)−W_(s)+1, and the end of the range asn_(c)−W_(s)−1, wherein n_(c)=n_(max) if n_(max) is for the first signalor n_(c)=n_(max)−d if n_(max) is for one of the other signals.

The first and other signals may be transmitted over channels having achannel width corresponding to less than d.

The received signal may comprise a first signal transmitted from a firsttransmit antenna and a plurality of shifted signals transmitted from aplurality of other transmit antennas, each shifted signal being shiftedby a different predetermined amount time from the first signal, and thefirst and other signals being received at a corresponding number ofreceive antennas, the method further comprising: decoupling the channelimpulse response for the channels between each transmit antenna and eachreceive antenna; and summing the decoupled channel impulse responses,the step of determining an energy value for each of a plurality of setsbeing carried out on the summed channel impulse responses.

The received signal may be an OFDM signal.

The received signal may be a signal in accordance with an IEEE 802.11protocol.

According to a second aspect there is provided a receiver comprising: anantenna configured to receive a signal representing a data packets,wherein each data packet comprises a guard interval preceding a preamblehaving a predetermined sequence of symbols; a sampler configured tosample the received signal at a sampling rate; and a symbol boundaryestimator configured to: estimate channel impulse responses from a setof samples in dependence on the predetermined sequence of symbols of thepreamble; determine an energy value for each of a plurality of windowsof channel impulse responses, each of the windows corresponding to Wnumber of consecutive samples, the energy value for each of the windowsbeing indicative of the total energy associated with the channel impulseresponses of that window; determine which of the windows has thegreatest estimated energy value; and identify the earliest sample of theconsecutive W samples in the determined greatest energy window, theearliest sample being indicative of a symbol boundary for the preamble.

The plurality of windows may comprise successive windows of W samples,each successive window being offset by one sample from a previous windowin the succession.

W may be dependent on the sampling rate and a duration of the guardinterval.

The guard interval duration may be 0.8 μs or 0.4 μs.

The data packet may further comprise a training field preceding theguard interval, the symbol boundary estimator being further configuredto estimate a boundary between the training field and the guardinterval, the set of samples being taken from the estimated boundary fora period corresponding to the duration of the preamble.

The receiver may further comprise one or more other receive antennas,wherein the received signal comprises a first signal transmitted fromone transmit antenna and one or more other signals transmitted fromrespective one or more other transmit antennas, the other signals beingthe same as the first signal and shifted by predetermined amount of timeup to a maximum shift equal to −d, the symbol boundary estimator beingfurther configured to: identify a sample n_(Igi) which is the earliestsample in the window of channel impulse responses of duration W havingthe greatest energy; calculate a sample range in dependence on samplen_(Igi), a peak sample n_(max) and a number Ws of samples correspondingto the duration of a short guard interval; determine an energy value foreach of a plurality of second windows of channel impulse responses, eachsecond window corresponding to Ws number of consecutive samples, theplurality of second windows being within the sample range, the energyvalue for each second window being indicative of the total energyassociated with the impulse responses of respective Ws samples;determine which second window has the greatest estimated energy value;and identify the earliest sample of the consecutive Ws samples in thedetermined second window, the earliest sample for the determined secondwindow being indicative of a symbol boundary.

W may correspond to a number of samples for the duration of a long guardinterval.

The symbol boundary detector may be configured to calculate the samplerange by: identifying the channel impulse response having the greatestenergy as n_(max); and determining if n_(max) is for the first signal orone of the other signals in dependence on n_(Igi) and W_(s).

The symbol boundary detector may be configured to calculate the samplerange by setting the start of the range as n_(c)−W_(s)+1, and the end ofthe range as n_(c)−W_(s)−1, wherein n_(c)=n_(max) if n_(max) is for thefirst signal or n_(c)=n_(max)−d if n_(max) is for one of the othersignals.

The first and other signals may be transmitted over channels having achannel width corresponding to less than d.

The received signal may comprise a first signal transmitted from a firsttransmit antenna and a plurality of shifted signals transmitted from aplurality of other transmit antennas, each shifted signal being shiftedby a different predetermined amount time from the first signal, and thereceiver comprising a corresponding number of receive antennasconfigured to receive the first and other signals, the symbol boundaryestimator being further configured to: decouple the channel impulseresponse for the channels between each transmit antenna and each receiveantenna; and sum the decoupled channel impulse responses, the step ofdetermining an energy value for each of a plurality of sets beingcarried out on the summed channel impulse responses.

The receiver may be configured to receive OFDM signals.

The receiver may be configured to operate in accordance with an IEEE802.11 protocol.

According to a third aspect there is provided machine readable code forimplementing the method described above.

According to a fourth aspect there is provided a machine readablestorage medium having encoded thereon non-transitory machine-readablecode implementing the method described above.

DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 shows a structure of a packet in accordance with the IEEE 802.11standard;

FIG. 2 shows a schematic diagram of a receiving device;

FIG. 3 shows an example channel impulse response for a single-inputsingle-output (SISO) system;

FIG. 4 shows an example channel impulse response for system with 2transmit antennas and N_(RX) receive antennas;

FIG. 5 shows an example channel impulse response for a wide band channelin a system with 2 transmit antennas and N_(RX) receive antennas; and

FIG. 6 shows an example of summing channel impulse responses for a wideband channel in a system with N_(Tx) transmit antennas and N_(Rx)receive antennas.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application. Various modifications to the disclosedembodiments will be readily apparent to those skilled in the art.

The general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles and features disclosed herein.

Examples described herein provide a new, energy based method ofdetermining a symbol boundary in an OFDM system with preamble that has apredetermined sequence of symbols. For a received signal, a coarseestimate of the symbol boundary may first be made using a method such asan auto-correlation based boundary estimation. Channel impulse responsescan be calculated from samples of the received signal using thepredetermined preamble symbols. The energy for each of a plurality ofwindows of channel impulse responses can be determined, wherein eachwindow corresponds to W number of consecutive samples. The energy valuefor each window is indicative of the total energy associated with thatwindow. In examples described herein, the window that has the greatestestimated energy value is determined and the earliest sample of theconsecutive W samples in the greatest energy window is identified. Theearliest sample is indicative of the symbol boundary. As described inmore detail below, this method allows the symbol boundary to bedetermined for packets transmitted in single-input single-output systemsand in multiple-input multiple-output systems operating with narrowbandor wideband channels.

FIG. 2 shows a receiving device 200 comprising an RF receiver unit 201,an analogue-to-digital converter (ADC) 202, a coarse boundary estimator203, a symbol aligner 204, a fast Fourier transform (FFT) unit 205, apacket processor 206 and a fine symbol boundary (FSB) estimator 207.

The RF receiver unit 201 receives an analogue OFDM signal and providesit to the ADC 202. The ADC 202 samples the analogue OFDM signal andfeeds it to the symbol aligner 204. The symbol aligner 204 aligns thesamples so that the input to FFT unit 205 is the 3.2 μs of samples foreach data symbol. The sample rate of the ADC 202 may be dependent on thebandwidth of the signal to be received, which may be, for example, 20,40, 80 or 160 MHz. The sampled signal is also fed to the coarse boundaryestimator 203, which provides a coarse estimate of the symbol boundaryfor the received signal and provides that estimate to the symbol aligner204.

The symbol aligner 204 aligns the samples initially according to thecoarse estimated symbol boundary from the coarse boundary estimator 203.The symbol aligner 204 feeds the aligned samples to the FFT unit 205.The FFT unit 205 computes the discrete Fourier transform (DFT) of thesamples. The DFT of the samples is fed back into the FSB estimator 207so that the FSB can be estimated. The FSB estimate is provided to thesymbol aligner 204 which aligns the symbols according to that estimate.The FFT unit 205 transforms the signal into a frequency domain signal,from which the payload data can be obtained via further processing atthe packet processor 206.

The correct start position (i.e., the symbol boundary) of the timedomain sampled signal should be accurately estimated so that the FFT isperformed at the proper time so as to avoid ISI effectively. The coarseboundary estimator 203 determines an estimate of the symbol boundary andprovides it to the symbol aligner 204 so that the samples can beproperly aligned for the FFT.

The plurality of time-sequential samples obtained from ADC 202 isprovided to the coarse boundary estimator 203, which coarsely estimatesthe time of the S10-DGI boundary. This coarse estimation can be carriedout using any method known in the art, such as auto-correlation usingthe short preamble.

From the coarse estimation of the S10-DGI boundary, a rough estimationof the time of the DGI-L1 boundary can be determined. The FSB estimator207 utilises 3.2 μs of samples from the time of the estimated S10-DGIboundary to provide a finer estimation of the time of the DGI-L1boundary. An N-point discrete Fourier transform (DFT) is performed onthese samples by the FFT unit 205 to convert the sampled signal from thetime domain to the frequency domain, where N is the total number oftime-domain samples of the signal in the 3.2 μs period.

The long preamble L1 has a predetermined sequence of symbols (which isdefined by the appropriate 802.11 standard) and so the long preamble isknown by the receiving device 200. Thus the long preamble signal in thefrequency domain is also known by the receiver. The FSB estimator 207compensates each of the frequency domain samples with the knownfrequency domain values of the long preamble signal. Each frequencydomain sample is compensated by multiplying the N-point DFT output forthe sample with the conjugate of frequency domain values of the knownlong preamble.

The following equations and steps below generally describe the procedurecarried out by the FSB estimator 207 to determine the DGI-L1 boundary.

For n=1, 2, . . . , N samples,

y(n)=h(n)

_(N) x(n)+w(n)

where y(n) is the received signal in the time-domain, h(n) is thechannel in the time-domain, x(n) is the transmitted signal in thetime-domain and w(n) is the noise signal in the time-domain.

Assuming the coarse boundary estimation is off by M samples, thereceived signal is the M samples delayed version of the long preamblesignal p(n), so

y(n)=h(n)

_(N) p(n−M)+w(n)

DFT is applied for k=1, 2, . . . , N

${z(k)} = {{{g(k)}{p(k)}^{\frac{{- {j2\pi}}\; {Mk}}{N}}} + {w(k)}}$

where g(k) is the channel in the frequency-domain, p(k) is the longpreamble in the frequency-domain, z(k) is the obtained signal in thefrequency domain and w(k) is noise.

The known frequency domain long preamble p(k) can be compensated in theabove equation by multiplication of both sides by the conjugate of p(k).

Hence,

${{z(k)}{p^{*}(k)}} = {{{g(k)}{p(k)}{p^{*}(k)}^{\frac{{- {j2\pi}}\; {Mk}}{N}}} + {{w(k)}{p^{*}(k)}}}$

As p(k)p*(k)=1, we get:

${\overset{\sim}{z}(k)} = {{{g(k)}^{\frac{{- {j2\pi}}\; {Mk}}{N}}} + {\overset{\sim}{w}(k)}}$

where {tilde over (z)}(k) and {tilde over (w)}(k) are intermediatesignals, with {tilde over (z)}(k)=z(k)p*(k) and {tilde over (w)}(k)=w(k)p*(k).

An inverse DFT (IDFT) is applied on both sides of above equation. Forn=1, 2, . . . , N, this results in

c(n)=h(n−M)+{tilde over (w)}(n)

where c(n) is the estimated channel in the time domain, which isestimated using the known long preamble. In a noise-free environment,c(n) is the shifted version of h(n). From the estimated channel c(n),the FSB can be found as described below with reference to FIG. 3.

FIG. 3 shows an example of the channel impulse response in asingle-input single-output (SISO) system. The graph shows the energy ofN samples (or channel taps). The channel taps represent impulseresponses of channels.

Since for an ISI-free OFDM system the number of channel taps withsignificant energy (e.g. taps that have energy that ≧90% higher than therest of the taps) should be less than the number of samples for theguard interval, a sliding window 301 is used to measure the energy of anumber of taps within a duration (i.e. the window size) that correspondsto the duration of the GI.

As shown in FIG. 3, the window 301 moves along the x-axis one tap at atime so that the energy of the channel taps in the GI-sized window canbe determined. The window position 302 with the greatest energy isdetermined. The first (i.e. earliest) tap 303 in the window 302 isselected to be an estimate of the position of the FSB. Selecting theearliest tap 303 as the FSB helps to mitigate the effect of ISI for L1.

In multiple-input-multiple-output (MIMO) OFDM systems, when transmittingand receiving with multiple antennas, the receiving device can sufferfrom undesirable beam forming if every antenna transmits the samesignal. In IEEE 802.11 systems, the preambles transmitted by differentantennas are shifted cyclically. This leads to additional complicationswhen determining the FSB due to the use of multiple cyclically shiftingsignals.

For example, in a 2×N_(rx) MIMO system (i.e. 2 transmitters and 1 ormore receivers), both transmitters transmit the same long preamblesignal but with a cyclic shift (d) (in the time domain) between thefirst and second transmitters. The cyclic shift causes the estimatedchannel for i^(th) receive antenna c_(i)(n) to contain the channelinformation of both transmitters with cyclic shift d. The estimatedchannel can be given by

For i ε [1 N_(rx)] and n=1, 2, . . . , N

y _(i)(n)=h _(i1)(n)

_(N) x ₁(n)+h _(i2)(n)

_(N) x ₂(n)+w _(i)(n)

where y_(i)(n) is the received signal in the time-domain, h_(i1)(n) andh_(i2) (n) are the channels in the time-domain between the first andsecond antennas of the transmitter and the i^(th) receiver respectively,x₁(n) and x₂(n) are the transmitted signals in the time-domain by thefirst and second transmitters respectively and w_(i)(n) is the noisesignal in the time-domain.

The received signal is the M samples delayed version of the longpreamble signal p(n) and there is a cyclic shift d between the signalstransmitted by the first and second transmitters, so:

y _(i)(n)=h _(i1)(n)

_(N) p(n−M)+h _(i2)(n)

p(n−M+d)+w _(i)(n)

Similarly to above, DFT is applied to the signal, the frequency-domainsignal is then compensated using the known long preamble signal in thefrequency-domain and an IDFT is applied to the estimated channel togive:

c _(i)(n)=h _(i1)(n−M)+h _(i2)(n−M+d)+{tilde over (w)}(n)

FIG. 4 shows a channel impulse response for an example of a narrow bandchannel in a MIMO system with 2 transmit antennas, where the shift delayis −d. As shown, in a noise free environment, two peaks at M and M-d areobtained.

The method described above of determining the highest energy window andthe earliest tap in that window can be used to estimate the FSB forstandard packets where the GI is longer than d. In this case, as shownin FIG. 4, the window 401 size is greater than d and so is able to coverboth M and M-d peaks. However, in the scenario of HT or VHT packets witha short GI, the short GI may be less than d and so the shorter windowsize 402 corresponding to the short GI may not be sufficient to coverboth peaks. Thus, if the peak at M-d is greater than the peak at M, thenthe window doesn't cover the peak at M, giving a false FSB reading. Toaccount for this, the following procedure is used for the case of a2×N_(rx) MIMO and HT or VHT packets.

The maximum energy peak tap location n_(max) is determined from the sumof absolute square of resultant channel Σ_(i)|c_(i)(n)|². Also, usingthe method described above in relation for FIG. 3, the tap locationn_(LGI) of the FSB using a window size for the long GI is determined.The following equation determines whether the peak at n_(max) belongs toΣ_(i)|h_(i1)(n)|² or Σ_(i)|h_(i2)(n)|² (i.e. the channel between thefirst transmitter antenna and the i^(th) receive antenna or the channelbetween the second transmitter antenna and the i^(th) receive antennarespectively):

${n_{LGI} - n_{\max}} = \left\{ \begin{matrix}{\geq {{W_{S}{peak}}\mspace{14mu} {at}\mspace{14mu} M\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {maximum}}} \\{< {{W_{S}{peak}}\mspace{14mu} {at}\mspace{14mu} M\text{-}d\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {maximum}}}\end{matrix} \right.$

where W_(S) is the number of taps (i.e. samples) in the short GI windowof 0.4 μs.

Once the peak location has been determined, the method described abovefor determining the FSB is carried out for a range of samples betweenn_(start) and n_(end), which are defined as:

n _(start) =n _(c) −W _(S)+1

n _(end) =n _(c) W _(S)−1

where n_(c) is the position of the highest energy peak M for channelbetween first antenna and the i^(th) receive antenna and is determinedby:

$n_{C} = \left\{ \begin{matrix}{{{n_{\max}\mspace{14mu} {if}\mspace{14mu} N_{LGI}} - n_{\max}} \geq {W_{S} - 1}} \\{{n_{\max} + {d\mspace{14mu} {if}\mspace{14mu} N_{LGI}} - n_{\max}} < W_{S}}\end{matrix} \right.$

The window of short GI size moves along the x-axis one tap at a timefrom n_(start) to n_(end) to determine the energy of W_(S) taps withinthat range. The window position with the greatest energy is determinedand the first (i.e. earliest) tap in that window corresponds to thelocation of the FSB for the HT or VHT packet transmitted by the firstantenna of the transmitter. By carrying out the FSB determination withinthe n_(start) and n_(end) range, ISI from the channel between the secondtransmit antenna and the i^(th) receive antenna can be avoided to give amore accurate FSB location.

In certain cases where a wide band channel is used in a 2×N_(Rx) MIMO,the channels between the first and second transmit antennas and thei^(th) receive antenna can contain multiple paths. Thus, the channelscan contain precursor, post-cursor and line of sight components. FIG. 5shows an example of this, where the channels for the first and secondtransmit antennas Σ_(i)|h_(i1)(n−M)|² and Σ_(i)|h_(i2)(n−M+d)|²respectively have 2 precursors, 3 post-cursors and a line of sightcomponent. In this case n_(start) is decided based on the number ofprecursors that are required to be covered and the FSB is determined asdescribed above.

However, if the width of the channels is greater than the number ofsamples corresponding to the shift d, then the channels will overlapwith each other, which may affect the FSB detection. Thus prior tocarrying out the FSB detection using this method, the FSB estimator 207may detect the channel type and width and determine the number ofprecursors required to be covered.

In the case of a N_(Tx)×N_(Rx) MIMO systems for narrow- and wide-bandchannels, additional LTFs are transmitted which are used to computechannel estimates h_(ij) ^(E)(n), where i=1, 2, . . . , N_(Rx) and j=1,2, . . . , N_(Tx). The channels h_(ij) ^(E)(n) are decoupled channelsand the methodology described above in relation to FIG. 3 can be appliedto the sum of absolute square of decoupled channel Σ_(i)|h_(i1)^(E)(n)|² to determine the FSB for h_(i1) ^(E)(n).

Alternatively, in the case of N_(Tx)×N_(Rx) MIMO systems, rather thandetermining the FSB for h_(i1) ^(E)(n), the shifted versions of theestimated channels h_(ij) ^(E)(n) are summed. The sum of the absolutesquare of resultant channel |c_(ij) ^(E)(n)|² is given by

${{c_{ij}^{E}(n)}}^{2} = {\sum\limits_{j}\; {\sum\limits_{i}\; {{h_{ij}^{E}\left( {n - d_{cj}} \right)}_{N}}^{2}}}$

where N is the duration of the long preamble and d_(j) is the cyclicshift for the j^(th) transmit antenna. The values for d_(j) arepredefined values, for example as defined in the IEEE 802.11 standard.|c_(ij) ^(E)(n)|² for an example signal is shown in FIG. 6. The FSB forthe resultant channel |c_(ij) ^(E)(n)|² can be found using the slidingwindow method described above.

The receiving device configured in accordance with the examplesdescribed herein could be embodied in hardware, software or any suitablecombination of hardware and software. The receiving device of theexamples described herein could comprise, for example, software forexecution at one or more processors (such as at a CPU and/or GPU),and/or one or more dedicated processors (such as ASICs), and/or one ormore programmable processors (such as FPGAs) suitably programmed so asto provide functionalities of the data processing system, and/orheterogeneous processors comprising one or more dedicated, programmableand general purpose processing functionalities. In the examplesdescribed herein, the transmitting and receiving devices comprise one ormore processors and one or more memories having program code storedthereon, the data processors and the memories being such as to, incombination, provide the claimed data processing systems and/or performthe claimed methods.

Data processing units described herein (e.g. ADC 202, coarse boundaryestimator 203, symbol aligner 204, FFT unit 205, packet processor 206and FSB estimator 207) need not be provided as discrete units andrepresent functionalities that could (a) be combined in any manner, and(b) themselves comprise one or more data processing entities. Dataprocessing units could be provided by any suitable hardware or softwarefunctionalities, or combinations of hardware and softwarefunctionalities.

The term software as used herein includes executable code for processors(e.g. CPUs and/or GPUs), firmware, bytecode, programming language codesuch as C or OpenCL, and modules for reconfigurable logic devices suchas FPGAs. Machine-readable code includes software and code for defininghardware, such as register transfer level (RTL) code as might begenerated in Verilog or VHDL.

Any one or more of the data processing methods described herein could beperformed by one or more physical processing units executing programcode that causes the unit(s) to perform the data processing methods.Each physical processing unit could be any suitable processor, such as aCPU or GPU (or a core thereof), or fixed function or programmablehardware. The program code could be stored in non-transitory form at amachine readable medium such as an integrated circuit memory, or opticalor magnetic storage. A machine readable medium might comprise severalmemories, such as on-chip memories, computer working memories, andnon-volatile storage devices.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

1. A method for detecting a symbol boundary between symbol intervals ina data packet comprising a guard interval preceding a preamble having apredetermined sequence of symbols, the method comprising: receiving asignal representing a data packet; sampling the received signal at aspecified sampling rate to obtain a set of samples; estimating channelimpulse responses from the set of samples in dependence on thepredetermined sequence of symbols of the preamble; determining an energyvalue for each of a plurality of first windows of channel impulseresponses, each of the first windows corresponding to W number ofconsecutive samples, W being an integer greater than 1, the energy valuefor each of the windows being indicative of the total energy associatedwith the channel impulse responses of that window; determining which ofthe first windows has the greatest energy value; and identifying theearliest sample of the consecutive W samples in said determined greatestenergy first window, the earliest sample being indicative of a symbolboundary in said data packet.
 2. A method as claimed in claim 1, whereinW is dependent on the sampling rate and a duration of the guardinterval.
 3. A method as claimed in claim 1, wherein the received signalcomprises a first signal transmitted from one antenna and one or moreother signals transmitted from respective one or more other antennas,said other signals being the same as the first signal and shifted bypredetermined amount of time, the method further comprising the stepsof: identifying the earliest sample n_(Igi) of the consecutive W samplesin the determined greatest energy first window; calculating a samplerange in dependence on sample n_(Igi), a peak sample n_(max) and a W_(s)number of samples corresponding to the duration of a short guardinterval; determining an energy value for each of a plurality of secondwindows of channel impulse responses, each second window correspondingto W_(s) number of consecutive samples, the plurality of second windowsbeing within the sample range, the energy value for each second windowbeing indicative of the total energy associated with the channel impulseresponses of respective W_(s) samples; determining which second windowhas the greatest estimated energy value; identifying the earliest sampleof the consecutive W_(s) samples in said determined second window, theearliest sample being indicative of a symbol boundary.
 4. A method asclaimed in claim 3, wherein W corresponds to a number of samples for theduration of a long guard interval.
 5. A method as claimed in claim 3,wherein said step of calculating a sample range comprises: identifyingthe channel impulse response having the greatest energy as n_(max); anddetermining if n_(max) is for the first signal or the one of the othersignals in dependence on n_(Igi) and W_(s).
 6. A method as claimed inclaim 5, wherein the maximum magnitude of the shift is d and said stepof calculating the sample range comprises setting the start of the rangeas n_(c)−W_(s)+1, and the end of the range as n_(c)−W_(s)−1, whereinn_(c)=n_(max) if n_(max) is for the first signal or n_(c)=n_(max)−d ifn_(max) is for one of the other signals.
 7. A method as claimed in claim6, wherein the first and other signals are transmitted over channelshaving a channel width corresponding to less than d.
 8. A method asclaimed in claim 1, wherein the received signal comprises a first signaltransmitted from a first transmit antenna and a plurality of shiftedsignals transmitted from a plurality of other transmit antennas, eachshifted signal being shifted by a different predetermined amount timefrom the first signal, and the first and other signals being received ata corresponding number of receive antennas, the method furthercomprising: decoupling the channel impulse response for the channelsbetween each transmit antenna and each receive antenna; and summing thedecoupled channel impulse responses, said step of determining an energyvalue for each of a plurality of sets being carried out on the summedchannel impulse responses.
 9. A receiver comprising: an antennaconfigured to receive a signal representing a data packet, wherein eachdata packet comprises a guard interval preceding a preamble having apredetermined sequence of symbols; a sampler configured to sample thereceived signal at a specified sampling rate to obtain a set of samples;and a symbol boundary estimator configured to: estimate channel impulseresponses from the set of samples in dependence on the predeterminedsequence of symbols of the preamble; determine an energy value for eachof a plurality of first windows of channel impulse responses, each ofthe first windows corresponding to W number of consecutive samples, Wbeing an integer greater than 1, the energy value for each of the firstwindows being indicative of the total energy associated with the channelimpulse responses of that window; determine which of the first windowshas the greatest energy value; and identify the earliest sample of theconsecutive W samples in said determined greatest energy first window,the earliest sample being indicative of a symbol boundary in the datapacket.
 10. A receiver as claimed in claim 9, wherein the plurality offirst windows comprises successive first windows of W samples, eachsuccessive first window being offset by one sample from a previous firstwindow in the succession.
 11. A receiver as claimed in claim 9, whereinW is dependent on the sampling rate and a duration of the guardinterval.
 12. A receiver as claimed in claim 9, wherein the guardinterval duration is 0.8 μs or 0.4 μs.
 13. A receiver as claimed inclaim 9, wherein the data packet further comprises a training fieldpreceding the guard interval, the symbol boundary estimator beingfurther configured to estimate a boundary between the training field andthe guard interval, the set of samples being taken from said estimatedboundary for a period corresponding to the duration of the preamble. 14.A receiver as claimed in claim 9 further comprising one or more otherreceive antennas, wherein the received signal comprises a first signaltransmitted from one transmit antenna and one or more other signalstransmitted from respective one or more other transmit antennas, saidother signals being the same as the first signal and shifted bypredetermined amount of time, the symbol boundary estimator beingfurther configured to: identify the earliest sample n_(Igi) of theconsecutive W samples the determined greatest energy first window;calculate a sample range in dependence on sample n_(Igi), a peak samplen_(max) and a number W_(s) of samples corresponding to the duration of ashort guard interval; determine an energy value for each of a pluralityof second windows of channel impulse responses, each second windowcorresponding to W_(s) number of consecutive samples, the plurality ofsecond windows being within the sample range, the energy value for eachsecond window being indicative of the total energy associated with theimpulse responses of respective W_(s) samples; determine which secondwindow has the greatest estimated energy value; and identify theearliest sample of the consecutive W_(s) samples in said determinedsecond window, the earliest sample being indicative of a symbolboundary.
 15. A receiver as claimed in claim 14, wherein W correspondsto a number of samples for the duration of a long guard interval.
 16. Areceiver as claimed in claim 14, wherein the symbol boundary detector isconfigured to calculate the sample range by: identifying the channelimpulse response having the greatest energy as n_(max); and determiningif n_(max) is for the first signal or one of the other signals independence on n_(Igi) and W_(s).
 17. A receiver as claimed in claim 16,wherein the maximum magnitude of the shift is d and the symbol boundarydetector is configured to calculate the sample range by setting thestart of the range as n_(c)−W_(s)+1, and the end of the range asn_(c)−W_(s)−1, wherein n_(c)=n_(max) if n_(max) is for the first signalor n_(c)=n_(max)−d if n_(max) is for one of the other signals.
 18. Areceiver as claimed in claim 14, wherein the maximum magnitude of theshift is d and the first and other signals are transmitted over channelshaving a channel width corresponding to less than d.
 19. A receiver asclaimed in claim 9, wherein the received signal comprises a first signaltransmitted from a first transmit antenna and a plurality of shiftedsignals transmitted from a plurality of other transmit antennas, eachshifted signal being shifted by a different predetermined amount timefrom the first signal, and the receiver comprising a correspondingnumber of receive antennas configured to receive the first and othersignals, the symbol boundary estimator being further configured to:decouple the channel impulse response for the channels between eachtransmit antenna and each receive antenna; and sum the decoupled channelimpulse responses, said step of determining an energy value for each ofa plurality of sets being carried out on the summed channel impulseresponses.
 20. A non-transitory computer readable storage medium havingstored therein processor executable instructions that when executedcause at least one processor to: receive a signal representing a datapacket; sample the received signal at a specified sampling rate;estimate channel impulse responses from a set of samples in dependenceon the predetermined sequence of symbols of the preamble; determine anenergy value for each of a plurality of windows of channel impulseresponses, each of the windows corresponding to W number of consecutivesamples, where W is an integer greater than one, the energy value foreach of the windows being indicative of the total energy associated withthe channel impulse responses of that window; determine which of thewindows has the greatest energy value; and identify the earliest sampleof the consecutive W samples in said determined greatest energy window,the earliest sample being indicative of a symbol boundary in the datapacket.